I'm not sure about being "reef unsafe" but bigger hermits are annoying. They knock over and dislodge smaller coral pieces... I find corals laying on each other and half dead after stinging each other... big hermits may also hunt smaller hermits. I got a clean up crew as part of a package from a tank shut down deal along with mixed hermits... when I got some new smaller hermits, two older big ones were repeatedly seen holding new hermit shells and trying to pull out their contents. The two big hermits went for a flight out the window and into the woods... haven't been a problem since.
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